Hey Norman - the man that never sleeps,
I've often heard the design that you are refering to as the "swing axle" design as opposed to "swing-arm" design. Perhaps there are a couple of terms going around for the traditional engine-on-swing arm design. "Swing axle" is also the term used to describe the crude independant rear suspension systems of some 1950s/60s sports cars, so maybe everyone is confused... :?
To my knowledge, the SivlerWing and the TMax both have their engines located in the frames, as opposed to on the swing arm like the European offerings, and other single cylinder big-scooters. People sometimes discount the SW from the engine-in-frame catagory because the position of the rubber mounted engine is rearward in the frame, and the swing arm pivots on the same axle as the crankshaft, whereas the TMax and the AN650's engines are centrally located (with the AN650's being by the far the furthest forward in the frame).
You are right that the engine-on-swing-arm design causes handling problems above 65mph, and thee would be very noticebale at 80mph plus. This is due to the high amount of unsprung weight that doesn't allow the suspension to adequately control the attutde of the motorcycle in highspeed bends. The frames used with these types of scooter also allow an amount of flex which attributes to the weave you have described when travelling at speed.
Yamaha have tried to counter tride to counter this with their new Majesty by producing a aluminium girder like frame for the all-new model this year, which will reduce the frame flex, and have fitted heavy duty rear suspension to delay the inevitable effects of the engine-on-swing-arm design.
For vehicles with top speeds of 85mph or so, I think the handling package is more than adequate, and I would go as far as saying that the engine-on-swing-arm design allows easy changes of direction at speeds below 70mph, especially at low speeds were the inherant low centre of gravity of this design means that it's almost impossible to drop these things. Similarly, beacuse the engine is located on the swing arm as opposed to in the frame a simple frame can be used allowing a low weight design. The all-new 2004 Forza (Reflex) weighs 171kg dry, with traditional lightweight tubular frame. The 2004 new Majesty 250 weighs 194kg. Both have 23bhp to play with.
From my rides on the Majesty 250 and 400, I'm not conviced that the slight increased rigidity is the result of more handling orinetated stock suspension than the new Forza, or due the frame. I rode a 2003 Forza/Reflex with Honda's optional sports suspenson rear shock kit and it had a similar feeling on winding roads.
Anyway, take it easy out there when grinding centrestand at 85mph! :wink: